The speaker is drawn back again and again to someone they can't let go of. The first reason they give is oddly practical: the bridges between them haven't burned, so the way back is still open. Coming back is easy, so they keep doing it.
The last two lines change the tone. "Even after they have" admits that the connection might one day be destroyed, yet the speaker refuses to treat that as an ending. If the bridge is gone, they'll swim. That image makes the longing feel bigger and more stubborn, since swimming is slower, harder, and riskier than walking across.
The mood is tender but a little worrying. The speaker sounds devoted and hopeful, but also unable to walk away, even when the relationship may be damaged beyond repair. The poem leaves it open whether that persistence is romantic or self-defeating, and it may be both.